There is at least one user with two Creators on an Ultra, but I don't recall who. You'll get a quicker response on the freenode IRC #gentoo-sparc channel.

That said, with two Creators, if you use DRI, the kernel will do bad things to you. Please review https://bugs.gentoo.org/show_bug.cgi?id=79467.
There is an active patch attached to this bug. You should download the patch, apply it as in Comment #6 to the bug (except now it is {path-to-xorg-x11-6.8.2-r1-patch}, of course), ahd rebuild xorg-x11 with USE="xaa {your-other-use-options}"

I myself haven't heard how xinerama works with 2 Creators, but two independent screens should be OK. The first time you start X, the Creator/Elite driver sunffb initializes everything it can. If you have two monitors and set them both up, sunffb should drive them both. I don't have first-hand knowledge, though.